“If we expect to limit warming to something tolerable, such as the 2 degree Celsius threshold widely accepted as a political judgment, the time is very close where you have to stop temporizing and start achieving reductions that are both large and rapid,” Dr. Somerville said. He said he and the other scientists who wrote the report “are speaking for ourselves,” adding, “Unlike the I.P.C.C. process, there were no governments involved, no filters.”

The scientists were driven by a shared sense that world leaders still think there is plenty of time to talk things over, year after year, he said. “At these climate meetings, once the negotiations start, you get the sense that they might as well be debating steel tariffs,” he said. “We’re trying to say it really is urgent to get going.”

There is no mention of geo-engineering, which in other circles is getting more traction these days as a backstop strategy that is essential to assess, given the trajectories of emissions of greenhouse gases. Specific popular arguments by those who challenge the case for disruptive climate change are addressed in boxes. EnviroKnow has posted an audio recording of a press call with some of the report’s authors.

Here is the report’s take-home summary:

Surging greenhouse gas emissions: Global carbon dioxide emissions from fossil fuels in 2008 were nearly 40 percent higher than those in 1990. Even if global emission rates are stabilized at present-day levels, just 20 more years of emissions would give a 25 percent probability that warming exceeds 2°C, even with zero emissions after 2030.

Recent global temperatures demonstrate human-induced warming: Over the past 25 years temperatures have increased at a rate of 0.19°C per decade, in very good agreement with predictions based on greenhouse gas increases. Even over the past 10 years, despite a decrease in solar forcing, the trend continues to be one of warming. Natural, short-term fluctuations are occurring as usual, but there have been no significant changes in the underlying warming trend.

Acceleration of melting of ice-sheets, glaciers and ice-caps: A wide array of satellite and ice measurements now demonstrate beyond doubt that both the Greenland and Antarctic ice sheets are losing mass at an increasing rate. Melting of glaciers and ice caps in other parts of the world has also accelerated since 1990.

Rapid Arctic sea ice decline: Summertime melting of Arctic sea ice has accelerated far beyond the expectations of climate models. The area of sea ice melt during 2007-9 was about 40 percent greater than the average prediction from I.P.C.C. AR4 climate models.

Current sea level rise underestimated: Satellites show recent global average sea level rise (3.4 millimeters per year over the past 15 years) to be around 80 percent above past I.P.C.C. predictions. This acceleration in sea level rise is consistent with a doubling in contribution from melting of glaciers, ice caps and the Greenland and West-Antarctic ice sheets.

Sea level predictions revised: By 2100, global sea level is likely to rise at least twice as much as projected by the Working Group 1 of the I.P.C.C. AR4; for unmitigated emissions it may well exceed one meter. The upper limit has been estimated as about two meters sea level rise by 2100. Sea level will continue to rise for centuries after global temperatures have been stabilized, and several meters of sea level rise must be expected over the next few centuries.

Delay in action risks irreversible damage: Several vulnerable elements in the climate system (e.g. continental ice sheets, Amazon rain forest, West African monsoon and others) could be pushed towards abrupt or irreversible change if warming continues in a business-as-usual way throughout this century. The risk of transgressing critical thresholds (“tipping points”) increases strongly with ongoing climate change. Thus waiting for higher levels of scientific certainty could mean that
some tipping points will be crossed before they are recognized.

The turning point must come soon: If global warming is to be limited to a maximum of 2°C above preindustrial values, global emissions need to peak between 2015 and 2020 and then decline rapidly. To stabilize climate, a de-carbonized global society – with near-zero emissions of CO2 and other long-lived greenhouse gases – needs to be reached well within this century. More specifically, the average annual per-capita emissions will have to shrink to well under one metric ton CO2 by 2050. This is 80-95 percent below the per-capita emissions in developed nations in 2000.

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By 2050 or so, the human population is expected to pass nine billion. Those billions will be seeking food, water and other resources on a planet where humans are already shaping climate and the web of life. Dot Earth was created by Andrew Revkin in October 2007 -- in part with support from a John Simon Guggenheim Fellowship -- to explore ways to balance human needs and the planet's limits.